Instances

Instance Types

Except for UPS instance types, the cloud systems do not and will not have battery or generator backup, if we lose power that’s too bad. Individual physical nodes are single points of failure for the virtual instances running on them, at this point it is left as an exercise for
the user to implement HA Clustering across multiple VMs or to monitor
their services and restart them if they fail. If your system is a single
VM and is not fault tolerant be sure to use one of the ups instance
types.

OpenStack requires the use of predefined “Instance Types” also referred
to as “Flavors”. These define the virtual hardware including number of
cpus, memory size, size of the root disk and optionally additional
ephemeral disk space. We’ve defined instance types using the following
scheme:

s1.<N>core - N cores, N x 512m RAM, 10G root disk

m1.<N>core - N cores, N x 1024m RAM, 16G root disk

lg.<N>core - N cores, N x 2048m RAM, 32G root disk

xl.<N>core - N cores, N x 4096m RAM, 64G root disk

These standard types run on cluster nodes without redundant components
and without UPS power and are best suited to tasks where the uptime of
an individual component is not critical such as compute nodes or worker
nodes behind a load balancer (see LBaaS section).

There’s one special flavor for instances launched specifically for
creating image snapshots:

for.snapshot - 2 cores, 4096M RAM, 0G root disk

For a fixed total amount of resources, a greater number of smaller
instances (say a few cores and a few GB of RAM each) will be easier to
pack onto the available host machines than a smaller number of larger
instances (tens of cores or tens of GB of RAM).

Obviously it’s not really 0G, that is magic for whatever the minimum
size of the base image is. This is ideal for making snapshots as you
want them to be as small as possible.

It is possible to create custom instance types for specific projects, so
your project may have extra types available, ask someone in your group
why that is and what they are for. If you need a custom size created
email help@csail.mit.edu.